Flat display panel devices continues to receive much attention since they provide distinct advantages over conventional cathode ray tubes which are now the standard visual display device. U.S. Pat. No. 3,612,758, sets forth those advantages and discloses a flat display panel employing migration of color pigment particles of form an image on a matrix addressable panel. Specifically, the patented display utilizes a suspension of colored particles maintained in a thin layer of dyed dielectric liquid, enclosed between two electrodes, one of which is transparent. Upon the application of a D.C. voltage of suitable polarity between the electrodes, colored particles will move through the liquid toward the transparent electrode and deposit on it. The liquid is dyed a contrasting color to the colored particles such that only when the particles are deposited on, or in very close proximity to, the transparent electrode will the particles be visible. Otherwise, the color of the display as viewed through the transparent electrode will be that of the dyed dielectric liquid.
The flat panel display of U.S. Pat. No. 3,612,758 has several problems associated therewith which are difficult to solve. One problem relates to the usage of a dyed liquid; the dye tends to be absorbed on the colored particles and the transparent electrode, diminishing the contrast and appearance of the display. Particle settling over a period of time (due to gravity), particle agglomeration and clumping together over a period of time, and particle adherence to the transparent electrode pose additional problems. Another problem is that the particles must move substantial distances during display operation, thereby causing the display to operate with relative slowness. Also, because D.C. fields are used, it is probable that electro-chemical changes will take place in the display over extended periods of time.
Recent literature has proposed magnetic particle displays in lieu of cathode ray tube displays in order to overcome the size limitations, high voltage and high power requirements of cathode ray tube displays. The model magnetic particle display proposed uses tiny particles each of which is a plastic magnet comprised of ferrite powder held together by a suitable binder. The particles are about 0.8 millimeters in diameter, black in one hemisphere and silvered (or otherwise light reflective) in the other hemisphere. A magnetic field generated by the four conductors nearest a desired image spot controls the orientation of the particles proximate that spot such that their orientation with respect to a viewer conveys optical images by scattering of ambient light. Because of the close proximity of the conductors to other image spots, there arises a problem of discrimination, that is, the ability to address a chosen image spot without addressing other proximate image spots. In order to prevent clustering of the magnetic particles, the particles are individually encapsulated with small amounts of clear liquid in small, thin walled transparent spherical shells. The encapsulated particles are then cemented onto a suitable substrate to form the display panel. The requirements of a magnetic field to provide particle orientation, individual encapsulation of the particles, individual attachment of the particles to a suitable substrate, and discrimination problems present serious drawbacks to utilization of magnetic particle displays of the type described. Also, magnetic particle displays of the type described have neither an effective built-in threshold behavior nor an effective memory capability.